1. Field of the Invention
The present invention relates to a data conversion method for converting digital data to signals suitable for the recording system or the transmission channel used when recording or transmitting the digital data onto magnetic tape, and a recording/reproducing apparatus employing the data conversion method.
2. Description of Related Art
Prior art data conversion methods employed in magnetic recording/reproducing apparatus include for example, an 8/10 modulation method such as disclosed in "THE DAT CONFERENCE STANDARD" (issued June 1987). The 8/10 modulation method is a data conversion method in which digital data are partitioned into datawords of 8 bits each for conversion into 10-bit codewords. FIG. 1 is a circuit block diagram for explaining this his data conversion method, and FIG. 2 is a data conversion table used for the same. In FIG. 1, the reference numeral 1 designates an encoder for accepting eight-bit, digital data and a one-bit table select signal (Q') at its respective inputs and for outputting a total of 11 bits, i.e. a 10-bit codeword plus a one-bit signal (Q) for selecting the table for the next codeword. Further, the numeral 2 denotes a flip-flop for delaying the codeword table selected signal by one dataword. The encoder 1 includes a read-only memory (ROM) or the like which contains the data conversion table shown in FIG. 2, wherein codewords of CDS (Codeword Digital Sum)=0 are mapped on a one-to-one basis to 256 datawords from "00" to "FF" of hexadecimal numeral, while in the case of codewords of CDS .noteq.0, pairs of codewords, one with CDS=+2 and the other with CDS=-2, are each mapped to one dataword, the table of Q'=-1 consisting of codewords of CDS=+2 and the table of Q'=+1 consisting of codewords of CDS=-2. The table select signal (Q) is used to select the CDS (table) having the directed on that suppresses the description of charges in the codeword sequence.
The operation of the above circuit will now be explained with reference to the timing diagram of FIG. 3. In FIG. 3 the reference signs (a), (Q), and (b) correspond to inputs/outputs at the respective parts shown in FIG. 1 and the reference signs (c) and (d) respectively represent an output signal from an NRZI. converter (not shown) and a DSV (Digital Sum Variation) value at the end of codeword.
First, an eight-bit dataword "FF" is input to the encoder 1 along with the table select signal (Q')=-1, and consequently, the encoder 1 outputs a 10-bit codeword "1111101010" or CDS=+2 corresponding to "FF" for Q'=-1. At the same time the table select signal Q=-1 is output for the next codeword. The parallel 10-bit signal is then converted to a serial signal, after which the signals NRZI-modulalted. As a result, the DSV value at the end of the codeword becomes +2.
Next, when "00" input to the encoder 1 the encoder 1 outputs Q'=1 together with a 10-bit signal "0101010101" of CDS=0 corresponding to "00" for Q'=-1 which is produced by introducing a one-symbol delay in the previous output Q=-1. As a result, the DSV value at the end of the codeword after NRZI modulation remains at +2.
Next, when "11" is input to the encoder 1, the encoder 1 outputs Q=-1 together with a 10-bit signal of CDS=-2 corresponding to "11" for Q'=1. As a result the DSV value at the end of the codeword after NRZI modulation becomes zero. In this manner for each eight-bit dataword input to the encoder 1 a codeword to be output is selected from the table of either Q'=-1 or Q'=1 corresponding to the dataword on the basis of the table select signal output previously. The DSV at the end of each codeword after NRZI modulation can only take the value 0, +2 or -2. This means that the DSV dispersion is suppressed, as a result of which DC-free data conversion is realized.
As described above, according to the prior art data conversion method eight-bit data is converted to a 10-bit codeword of CDS=0 or CDS=+2 or -2 and a DC-free signal is produced with the DSV dispersion suppressed, thereby minimizing intersymbol interference on the transmission channel and thus increasing the recording density per track. However, for recent digital magnetic recording/reproducing apparatus using a rotary head, a recording density as high as several square micrometers per bit is demanded, which necessitates not only increasing the recording density per track but also reducing the track width down to several micrometers. To implement such apparatus, it is essential to employ a dynamic tracking following (DTF) control system whereby pilot signals for tracking are recorded on the main track recorded by the rotary head and the playback head is controlled to follow the recorded track curves during playback. When the prior are data conversion method is employed in such apparatus for multiplex recording of the pilot signals, the digtital signal spectral distribution has to be obtained down to ultra low frequency ranges although the recorded information signals contain no DC components; the resulting problem is that the pilot signals cause external disturbances, leading to increased errors in the detection of the digital signals.
One possible approach to overcoming the problem of the pilot signals causing external disturbances to the digital signals may be by generating pilot signals synchronized to the digital signals. However, the prior art data conversion method is effective only in suppressing the DSV dispersion and is not capable of actively controlling the DSV and therefore, has the problem that it cannot general pilot signals synchronized to the digital signals.
FIG. 4 shows a DAT recording format employed in a magnetic recording apparatus using the 8/10 modulation method. As shown, according to the format of FIG. 4 ATF areas for tracking control are provided in each of which a pilot signal for tracking control is recorded. Further, FIG. 5 shows a digital VTR recording format which is disclosed in Japanese Patent Application laid Open No. 3-217179(1991). As shown, the track is divided into a video data area, an audio data area, a servo pilot area, and a sub code area, the pilot signal being recorded in the servo pilot area only.
According to the above construction of the prior art, it is not possible to control the DSV in such a manner as desired, and a separate area has to be reserved for recording a pilot signal for tracking control. Accordingly, accurate tracking control cannot be realized without increasing the data amount and hence increasing the recording rate, which makes it difficult to achieve high density recording.